What is XMLHttpRequest's life cycle? When will the XMLHttpRequest be destroyed? - ajax

Question

When is XMLHttpRequest destroyed?
I just wrote the simple function below, and xmlhttp has been sent successfully, but responseText isn't sent to the function SetStringinDiv(). The variable str is empty and the variable location has value.
I have no idea how to fix it, and I don't know what the keyword for this problem is, either. If this kind of question already exists, please tell me how to find it or what the keyword is.
<script>
function foo(){
xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
var str = xmlhttp.responseText;
SetStringinDiv("div1", str);
}
}
xmlhttp.open("GET", "uri", true);
xmlhttp.send();
}
</script>
<script>
function SetStringinDiv(location, str) {
document.getElementById("location").innerHTML = location;
document.getElementById("str").innerHTML = str;
if (document.getElementByID(location) == null) {
document.getElementById("error").innerHTML += ">> error, can not find the div, "
+ location + "\n";
}
document.getElementByID(location).innerHTML = str;
}
</script>

Answer 1

Check for xmlhttp.status != 200 and print out an error. Print out the readystate and status on each call to onreadystatechange. I think you will see an error message in statusText.
These days it is recommended to not use the onreadystatechange callback. I use the code below. Note that if the server does not respond and does not close the connection that your request won't timeout for quite some time (2-5 minutes) - either in your code or that below. If it is sitting there, shut down the server. You will get an immediate error response then.
var req = new XMLHttpRequest(),
done = function(response) {
callback(cache[uri.split('/').pop()] = cache[uri] = response);
};
req.open("GET", uri + ".lispz", true);
req.onerror = function(err) {
done({
uri: uri,
error: err
});
}
req.onload = function() {
return (req.status != 200) ? req.onerror(req.statusText) : done(run(req.responseText));
};
req.send();